Introduction to Immunology

Introduction to the Immune System

• Immunology is the study of the immune system or immunity
• It involves the study of all aspects of host defense against infection and adverse consequences of immune responses
• The study of physiological mechanisms that enable the body to recognize materials as foreign and neutralize, metabolize or eliminate them without injury to the host tissue

Importance of the Immune System

• The importance of the immune system is illustrated by the observation that individuals with defective immune responses are susceptible to serious infections
• Stimulating immune responses against microbes through vaccination is the most effective method for protecting individuals against infections

Innate and Adaptive Immunity

• Host defenses are grouped into two categories: innate immunity and adaptive immunity
• Innate immunity provides immediate protection against microbial invasion and is always present in healthy individuals
• Adaptive immunity develops more slowly and provides more specialized defense against infections
• Innate immunity includes epithelial barriers, cells, and natural antibiotics present in epithelia, which function to block the entry of microbes
• Adaptive immune responses are mediated by lymphocytes with highly diverse and variable receptors for foreign substances

Properties of Adaptive Immune Responses

• Adaptive immune responses are specific, diverse, and self-limited
• They involve clonal selection, in which a clone of specific lymphocytes is stimulated to proliferate and differentiate in response to an antigen
• Memory cells are formed during adaptive immune responses, allowing for faster and more effective responses to subsequent exposure to the same antigen

Cells of the Adaptive Immune System

• Lymphocytes are the cells of the adaptive immune system
• There are two main types of lymphocytes: B lymphocytes and T lymphocytes
• B lymphocytes recognize soluble or microbial surface antigens and differentiate into antibody-secreting cells called plasma cells
• T lymphocytes recognize peptides derived from intracellular microbial proteins displayed on the surface of infected cells or antigen-presenting cells

Maturation and Tissue Distribution of Lymphocytes

• Lymphocytes develop from precursors in generative lymphoid organs (bone marrow and thymus)
• Mature lymphocytes enter peripheral lymphoid organs, where they respond to foreign antigens and recirculate in the blood and lymph

Stages in the Life History of Lymphocytes

• Lymphocytes go through three stages: naive, effector, and memory
• Naive lymphocytes recognize foreign antigens to initiate adaptive immune responses
• Effector cells function to eliminate antigens
• Memory cells differentiate into long-lived memory cells that can survive for long periods in the absence of antigen

Antigen-Presenting Cells

• Antigen-presenting cells (APCs) capture and process antigens, then present them to lymphocytes
• Dendritic cells are the most specialized APCs in the immune system

Tissues of the Immune System

• The tissues of the immune system include generative lymphoid organs (bone marrow and thymus) and peripheral lymphoid organs (lymph nodes, spleen, and mucosal and cutaneous immune systems)
• Peripheral lymphoid organs are organized to promote the development of adaptive immune responses

Peripheral Lymphoid Organs and Tissues

• Lymph nodes, spleen, and mucosal and cutaneous immune systems are peripheral lymphoid organs and tissues

• These organs are organized to trap and concentrate antigens, allowing for efficient recognition and response by lymphocytes### Lymphocyte Migration and Segregation

• Naive T and B lymphocytes migrate to different areas of a lymph node through a high endothelial venule (HEV) and are drawn to specific areas by chemokines that bind selectively to either cell type.

• B lymphocytes are attracted to and retained in the follicles due to the action of chemokines.

• Dendritic cells pick up antigens from epithelia, enter through afferent lymphatic vessels, and migrate to the T cell–rich areas of the node.

Lymphocyte Recirculation and Migration into Tissues

• Naive lymphocytes constantly recirculate between the blood and peripheral lymphoid organs, where they may be activated by antigens to become effector cells.
• Effector lymphocytes migrate from lymphoid tissues to sites of infection, where microbes are eliminated.
• Naive T lymphocytes migrate from the blood through high endothelial venules (HEVs) into the T cell zones of lymph nodes, where they are activated by antigens.
• Activated T cells exit the nodes, enter the bloodstream, and migrate preferentially to peripheral tissues at sites of infection and inflammation.

Immune System Functions

• The physiologic function of the immune system is to protect individuals against infections and cancers.
• Innate immunity is the early line of defense, mediated by cells and molecules that are always present and ready to eliminate infectious microbes.
• Adaptive immunity is mediated by lymphocytes stimulated by microbial antigens, which leads to the proliferation and differentiation of lymphocytes and generation of effector cells.

Lymphocyte Functions

• B lymphocytes are the only cells that produce antibodies, which recognize and neutralize antigens.
• T lymphocytes recognize peptide fragments of protein antigens displayed on other cells and activate phagocytes to destroy ingested microbes, recruit leukocytes, and activate B lymphocytes to produce antibodies.
• Cytotoxic T lymphocytes (CTLs) kill infected cells harboring microbes in the cytoplasm.

Immune Response Phases

• Adaptive immune responses consist of sequential phases: antigen recognition by lymphocytes, activation of the lymphocytes to proliferate and to differentiate into effector and memory cells, elimination of the microbes, decline of the immune response, and long-lived memory.